Conventional fly-back switch-mode power supply (SMPS), as shown in FIG. 1, includes a comparator, a logic control circuit, a driver circuit, a power switch and an inductor current sensing circuit. The inductor current of the SMPS is fed into the drain of the power switch and the source of the power switch is coupled to the inductor current sensing circuit. When the power switch turns on, the inductor current of the SMPS flows through the inductor current sensing circuit which provides a sensing voltage. The comparator compares the sensing voltage with a reference voltage. The output of the comparator is coupled to the logic control circuit. After being driven by the driver circuit, the output of the logic control circuit controls the gate of the power switch.
Generally, the inductor current of the SMPS, as illustrated in FIG. 1, is a slope current. Once the inductor current reaches a predetermined reference current level, the output of the comparator inverts and the power switch takes an action. Since there is a delay from the inversion of the comparator's output to the action taken by the power switch, a difference may occur between an actual inductor current turn-off value and the predetermined reference current. The difference is associated with the delay time of the power switch and the up-slope of the inductor current, while the up-slope of the inductance current is associated with a voltage drop across the inductor and the inductance of the inductor. In the case of a constant reference current, the actual inductor current peak may vary as the up-slope of the inductor current varies.
A waveform of the inductor current is illustrated in FIG. 2. By virtue of a sampling resistor R1, the inductor current I1 is converted into a sensing voltage Vsense which is then compared with the reference voltage VR. The reference current for the inductor current peak can be set as:
      I          pk      ⁢                          ⁢      0        =            VR              R        ⁢                                  ⁢        1              .  
The difference between the actual inductor current peak and the set reference current may be denoted as ΔIpk. The difference may be expressed as:
                              Δ          ⁢                                          ⁢                      I            pk                          =                                                            V                L                            L                        ·            Δ                    ⁢                                          ⁢                      t            d                                              (        1        )            
where VL denotes the voltage across the inductor, Δtd denotes a turn-off delay of the power switch, and L denotes the inductance of the inductor.
The actual inductor current peak is given as:Ipk=Ipk0+ΔIpk  (2)
The conventional SMPS operates under a constant reference current. The inductor current peak of the SMPS may vary with the voltage drop across the inductor and the inductance of the inductor. As the inductor current peak flowing through the SMPS is no longer constant, the SMPS may be damaged due to over-current. Thus, there exists a need to effectively control the inductor current peak, thereby effectively controlling the output voltage, the output current or the output power.